Treatment of titanium cathode surfaces

ABSTRACT

A METHOD OF EXTRACTING METAL FROM AN ELECTRLYTE IS DESCRIBED USING A PARTICULAR CATHODE WHICH HAS BEEN TREATED BY IMMERSION AND/OR ANODISATION IN A SUBSTANTIALLY FLUORIDE FREE ALKALINE AQUEOUS SOLUTION OF AN ORGANIC COMPLEXING AGENT FOR TILANIUM. THE CATHOD ITSELF IS ALSO DESCRIBED.   D R A W I N G

Aug. 13, 1914 G, M 's Em 3,829,366

TREATMENT OF TITANIUM CATHODE SURFACES Filed Oct. 27. 1972 "United Stat P nt one 3,829,366 Patented Aug. 13, 1974 ABSTRACT OF THE DISCLOSURE A method of extracting metal from an electrolyte is described using a particular cathode which has been treated by immersion and/or anodisation in a substantially fluoride free alkaline aqueous solution of an organic complexing agent for titanium. The cathode itself is also described.

BACKGROUND OF THE INVENTION In electrolytic refining of metals, an electrolyte containing ions of the metal to be refined is brought into contact with an anode and a cathode, a current is passed between the two electrodes and the metal is deposited on the cathode. The anode may be formed from the metal being refined, in which case it goes into solution at the same rate as the rate of deposition onto the cathode. Alternatively it may be formed from a passive material, and the deposition then being accompanied by a depletion of the metal ions from the solution.

The cathode can either be made from the metal which is being deposited or from some other metal. In copper refining, for example, the cathode can be a simple titanium sheet on which the copper is deposited. The copper may be thinly plated to form a thin sheet, which is subsequently removed and used as a starter blank. Alternatively the copper may be thickly deposited and then stripped off in slab form. In either case the deposition of the copper onto the titanium sheet must be uniform and the copper must be readily removable once deposition is complete. These two requirements are somewhat opposed, since ease of deposition would seem to entail a bonding between the cathode surface and the copper. The reason for the use of titanium or a titanium alloy for the cathode surface is its ability, to withstand corrosion in the normally acidic solutions used for electroplating.

SUMMARY OF THE INVENTION By the present invention there is provided a method of extracting a metal from an electrolyte which comprises contacting the lectrolyte with an anode and a cathode, and passing an electric current between the anode and the cathode, wherein the cathode has a surface of titanium or a titanium alloy, the surfacehaving been treated by insertion into a substantially fluoride-free alkaline aqueous solution and an organic complexing agent for titanium selected from the group: an ether, an alcohol, an aldehyde, a ketone, an amine, a nitrile, a sugar, a salt of an organic acid or any of their derivatives, the arrangement being such that a continuously thickening distorted anatase titanium oxide film is formed on the titanium or titanium alloy surface.

The complex formed by the complexing agent may be unstable in water. The solution may be at ambient temperature or at an elevated temperature. The organic complexing agent may be chosen from the group sodium gluconate, triethanolamine or ethylene diamino tetra acetic acid. In particular the solution may be chosen from (a) 25 wt. percent sodium hydroxide, 1-7 wt. percent sodium gluconate aqueous solution, or

(b) 10 wt. percent sodium hydroxide, 4 wt. percent triethanolamine, 2 wt. percent ethylene diamino tetra acetic acid aqueous solution, or

(c) 10 wt. percent sodium hydroxide, 4 wt. percent triethanolamine aqueous solution.

The oxide may be built up until it attains a black appearance corresponding to a minimum thickness of 750 A. to 3000 A. The alkali may be an alkaline metal hydroxide.

The surface of titanium or titanium alloy may be connected as an anode and a voltage may be passed through the anode during its insertion in the substantially fluoride free alkaline aqueous solution of the organic complexing agent for titanium. The anode voltage may be less than 12 v. and may be approximately 6 v. The solution may contain an alkaline metal hydroxide or ammonium hydroxide. The complex formed by the organic complexing agent may be unstable in water and the solution may be at ambient temperature or at an elevated temperature. The complexing agent and solutions may be chosen from the groups mentioned above.

The present invention also provides a cathode for an electrolytic cell, the cathode having a surface of titanium or a titanium alloy, the surface having been treated by insertion into a substantially fluoride-free alkaline aqueous solution including an organic complexing agent for titanium selected from the group consisting of an ether, an alcohol, an aldehyde, a ketone, an amine, a nitrile, a sugar, a salt of an organic acid or any of their derivatives, so that a thickened distorted anatase titanium oxide film has been formed on the titanium or titanium alloy surface.

The complex formed by the complexing agent may be unstable in water and the solution may be at ambient or at an elevated temperature. Again the organic complexing agents may be chosen from those detailed above as may the solutions.

The surface of the cathode may be connected as an anode and a voltage passed through the anode during its insertion in the substantially fluoride free alkaline aqueous solution of the organic complexing agent for titanium. The anode voltage may be less than 12 v. and may be approximately 6 v. The present invention further provides for the use of surface of titanium or a titanium alloy as a cathode in electrolytic metal extraction from an electrolyte characterised in that the surface has a thickened distorted anatase titanium oxide film on the surface produced by immersion of the surface in a substantially fluoride-free alkaline aqueous solution and an organic complexing agent for titanium selected from the group: an ether, an alcohol, an aldehyde, a ketone, an amine, a nitrile, a sugar, a salt of an organic acid, or any of their derivatives.

The complex formed by the complexing agent may be unstable in water and the solution may be at ambient or an elevated temperature. The organic complexingagen't may be chosen from those detailed above as may the solution. The surface of titanium or titanium alloy may be connected as an anode and a voltage passed through the anode during its insertion in a substantially fluoride free alkaline aqueous solution of the organic complexing agent for titanium. The anode voltage may be less than 12 v. and may be approximately 6 v.

BRIEF DESCRIPTION OF THE DRAWING By way of example, embodiments of the present invention will now be described with reference to the accompanying drawing which is a schematic cross-section of an electrolytic cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT The electrolytic cell comprises a bath 1 of concrete with a polyethylene liner 2, an electrolyte 3 in the bath contains an anode 4 and a cathode 5. The anode 4 is a copper anode and the electrolyte is a 20 wt. percent copper sulphate 10 wt. percent sulphuric acid aqueous solution.

The cathode 5 is a sheet of commercial purity titanium plate (designated IMI 130) which has been treated as follows:

A 25 wt. percent NaOH, 7 wt. percent sodium gluconate aqueous solution was prepared, heated to 60 C., and the piece of titanium was inserted and connected as an anode. A potential of 6 v. was established between the anode and a cathode. A film of oxide built up gradually and continuously. In the space of one hour, the film grew to an approximate thickness of 2000 A. and had a black appearance. This is in complete contrast to the expected result which would have been that the film would build up, and then as its resistance increases the rate of build up would be expected to slow rapidly until the thickness reaches a peak thickness dependent on the voltage.

Tests were done with a series of solutions at a series of temperatures and applied voltages. The results of the tests are shown in the accompanying Tables I to V. In the tables, the percentage is weight percent, the term immersion is used to denote soaking with no applied voltage, and the column plating details the ease and quality of copper deposit on the prepared surface.

The tables show that the growth of a suitable film is time, temperature and voltage dependent. The voltage applied has an upper limit dependent on the solution, but the upper limit tends to be the potential at which gas evolution occurs. The gas tends to agitate the solution. It appears that the mechanism of deposition of the film may be that the titanium on the plate is taken into solution in the form of a titanium complex which is basically unstable. Decomposition of the unstable complex releases titanium ions which are deposited on the surface. The disruptive effect of the gas bubbles interferes with the complexing and deposition arrangement and thus stops the growth of the film. It is found that voltages of 30 v. are normally disruptive.

Voltages in excess of 15, and normally over 12, are not preferred.

Temperatures up to 80 C. seem to have only the effect of speeding up the process. 60 C. is a preferred temperature because the current flow tends to maintain such a temperature and also to maintain temperatures in excess of 60 '0. becomes relatively expensive.

The reason for a titanium cathode with the film produced by the invention being so suitable is not completely clear. Without prejudice to the invention, it is believed that the thick film of the invention fills in mechanical defects on the surface, and hence reduces the mechanical keying effect of the copper or other metal eventually plated onto the surface. In addition, the more uniform surface of the invention produces a large number of nucleation sites of approximately equal, low free energy of formation. These many nucleation sites result in a small amount of sideways growth from the sites, and consequently an even growth with no'pinholes in the copper. Thus the surface of the invention is both easy to grow on because of the large number of nucleation sites,

and also the adhesion is low because of the reduced mechanical keying.

It has been discovered that the distorted anatase film grows over the original air formed TiO film present on the titanium sheet without significantly altering it. However, there is no change in the cell voltage using the cathode of the present invention instead of a simple titanium I sheet.

The black colour of the film is formed after passing through brown, dark green, red and dark blue. The character of the film is the same, only the thickness has changed.

In addition to under production of the film which can occur if treatment is not maintained for a sufiicient time, over thickening can also occur.

The anodisation for seven days at 6 v. 60 C. in a 25% NaOH 7% sodium gluconate solution produces eventually a white powdery film which has the insulating properties of bulk titanium dioxide. Thus over treatment must be avoided as must under treatment.

Trials with NaOH aqueous solutions and sodium gluconate aqueous solutions alone resulted in only poor oxide film. X-ray crystallography established that the oxide film is in part of a modified or distorted form of the anatase form of TiO and is relatively uniform in thickness and relatively free from porosity-4t is thought that the film has a defect lattice, a pure titanium oxide 1750 A. thick would be a green colour, not black as is experi enced.

Alternatively the cathode may be formed by mere immersion of the titanium into the NaOH/ sodium gluconate bath without anodisation, although the time needed is seven days. Again a black film built up on the surface, and its structure was proved to be a distorted anatase by X-ray crystallography.

A cathode of the coated titanium was then inserted into the cell. It was found that a very even coating of copper formed on the cathode, and that this copper coating stripped readily to form a sheet of copper suitable for a starter sheet. The titanium mother plate was substantially unaffected and was immediately suitable for re-use.

Such a titanium mother plate can be usefully substituted for the copper mother plates which have been in common use hitherto. In the production of copper by electrolytic refining, a supply of thin sheets of copper, known as starter sheets, is required which forms the cathodes onto which the copper is deposited. Electrolytic refiners can make their own copper starter sheets most cheaply by depositing electrolytically a layer of copper onto a plate and then stripping the plate after a certain thickness has built up. A release agent is used to' enable the copper starter sheets to be stripped from the mother plates. This release agent tends to contaminate the main electrolyte as it tends to be carried into the main electrolyte on the starter sheet.

Such a release agent is unnecessary with the titanium mother plate and hence this particular contaminant of the main electrolyte is removed.

It is envisaged that a surface coating produced according to the invention may be used on a drum or a continuous belt of titanium which acts as a cathode and which is passed through an electrolyte to have a suitable metal such as copper, nickel, or silver continuously electroplated onto the surface and continuously stripped ofl? to form a continuous strip of relatively thin metal. I g

It has been found that increasing the voltage above approximately 10-15 v. can result in a streaky oxide coating on the surface which is not so acceptable as the smoother coatings produced at lower voltages, although oxide growth does occur. a I

The cathodes are also particularly suitable for .cathode starters in gold electrolytic recovery, where :they can be substituted for the gold starter sheets used before, with a. great saving in the amount of capital tied up in the sheet.

wt" TABLEI 11..

6% NeOH, 6% Glycerol, 2% NaCl aqueous solution Treatment Film colour Plating Immersion: i

7 days, R.T Bronze. Neutral response. 7 days, 80 C.--. Mid-grey. Improved. Anodised:

7 days, 2 v., 20 0.. Do. 7 days, 2 v., 80 Not improved. 24 hrs., 6 v., 20 0-. Light brown Improved. 48 hrs., 6 v., 20 0.. Light straw Very good. 1 min., 30 v., 20 C. (rising) Streaky Poor.

H I TABLE ll v 25% NaQH,.7% Sodium Gluconate aqueous solution Treatment Film colour Plating Immersion: v V V 7 days, 20 C None Poor. 7 days, 80 C .Grey Moderate. Anodised:

7 days, 2 v., 20 C Black Good. 7 days, 2-v., 80 C. Moderate. 24 hrs, 6 v., 20 0... Very good. 48 hrs., 6 v., 20 0 Extremely good. 1hr.,6v., 0 0.- Good. 4 hrs 12 v 20 0 do. Very good.

12 hrs., 12 v., 20 C. Blue/black Extremely good. I }4 hr., 12 v., 20 C Purple/blue Poor strike, good deposit. 5 mins., 27 v., 20 C- Yellow/blue Moderate adhesion.

' TABLE .III

% NaOH, 4% Triethanolamiue, 2% EDTA Treatment Film colour Plating Immersion: I

7 days, 20 C Light green/red... Moderate. 7 days, 80 0.... Dark grey Good. Anodised;

' do Do.

Moderate. 24 hrs., 6 v., 20 C Bed/purple Very good 48 hrs., 6 v., 20 Brown/purple- Do. 4 hrs 12 v., 20 0.- Grey[b1ue Do. 12 hrs., 12 v., 20 C. Blue/black Excellent. 5 mins., 15-30 v., C Normal colour, Poor.

i v 1 no film growth.

I TABLEIV 10% NaQH, 4% Triethanolamine Treatment Film colour Plating Immersion:

7 days, 20 C Light grey. Good. 7 days, 80 C do D0.

Very good. 7 days, 2 v., 80 Good. 24 hrs.-,.6 v., 20 C Red/p 0.

" 48hrs., 6 v., 20 C Red/black Very good. rhrs 12 v., 20C Deep purple Moderate. 12 hrs., 12 v., 20 C. Blue/green Poor str ke, grey. deposit good.

5 mins., 2055v., 20 C No film thicken- Poor. i ng,

TABLE V Solution: Plating NaOH Poor EDTA Poor Sodium glucouate Poor We claim:

1. A method of extracting a. metal from an aqueous solution of a salt of the metal by electrolytic depos1t1on of the metal onto a cathode surface of a treated titanium surface of titanium or a titanium alloy, inserting the surface into a substantially [fluoride-free alkaline aqueous solution of' an organic complexing agentior titanium selected from the group consisting of an ether, an alcohol, an aldehyde, a ketone, an amine, a nitrile, a sugar, a salt of an organic acid and derivatives thereof, so that a continuously thickening distorted anatase titanium oxide film is formed on the titanium or titanium alloy surface, removing the surface from the solution, and at least once thereafter inserting the treated surface into the aqueous solution of the metal, connecting the treated surface as a cathode, electrodepositing the metal onto the surface and removing the electrodeposited metal from the surface.

2. A method as claimed in claim 1 in which the complex formed by the complexing agent is unstable in water.

3. A method as claimed in claim 1 in which the solution is at ambient temperature.

4. A method as claimed in claim 1 in which the organic complexing agent is chosen from the group consisting of sodium glucouate, triethanolamine, and ethylene diamino tetra acetic acid.

5. A method as claimed in claim 4 in which the solution is chosen from the group consisting of:

(a) 25 wt. percent sodium hydroxide, l-7 wt. percent sodium glucouate aqueous solution;

(b) 10 wt. percent sodium hydroxide, 4 wt. percent triethanolamine, 2 wt. percent ethylene diamino tetra acetic acid aqueous solution; and

(c) 10 Wt. percent hydroxide, 4 wt. percent triethanolamine aqueous solution.

6. A method as claimed in claim 1 in which the oxide is built up until it attains a black appearance corresponding to a minimum thickness of 1750 A. to 3000 A.

7. A method as claimed in claim 1 in which the alkali is an alkaline metal hydroxide.

8. A method as claimed in claim 1 in which the surface of titanium or a titanium alloy is connected as an anode, and a voltage is passed through the anode during its insertion in the substantially fluoride-free alkaline aqueous solution of the organic complexing agent for titanium.

9. A method as claimed in claim 8 in which the anode voltage is less than 12 v.

10. A method as claimed in claim 9 in which the voltage is approximately 6 v.

11. A method as claimed in claim 8 in which the solution contains an alkaline metal hydroxide or ammonium hydroxide.

12. A method as claimed in claim 8 in which the complex formed by the organic complexing agent is unstable in water.

13. A method as claimed in claim 8 in which the solution is at ambient temperature.

1.4. A method as claimed in claim 8 in which the organic complexing agent is chosen from the group consisting of sodium glucouate, triethauolamine, and ethylene diamino tetra acetic acid.

15. A method as claimed in claim 14 in which the solution is chosen from the group consisting of (a) 25 wt. percent sodium hydroxide, 1-7 wt. percent sodium glucouate aqueous solution,

(b) 10 wt. percent sodium hydroxide, 4 wt. percent triethanolamine, 2 wt. percent ethylene diamino tetra acetic acid aqueous solution, and

(c) 10 wt. percent sodium hydroxide, 4 wt. percent tri- I 7 deposited 'metalfrorn said surface, the improvement which comprises, using as the "cathode surface, one obtained by inserting a titanium or titanium alloy surface in a substantially fluoride-free alkaline aqueous solution of an organic complexing agent for titanium selected from the group consisting of an ether, analcohol, an aldehyde, a ketone, an amine, a nitrile, a sugar, a salt of an organic acid and derivatives thereof, so that a continuously thickening distorted anatase titanium oxide film is formed'on the titanium or titanium alloy surface.

18. The process of claim 17 wherein the metal is copper, nickel, silver or gold.

References Cited ["UNITEDSTATES PATENTS 1 2,646,396 7/1953 Dean 204 12 3,400,058 9/1968 Ross et al 204-56R 3,554,881 1/1971 Piontelli 2O4 56 R JOHN H. MACK, Primary Examiner V D. R. VALENTINE, Assistant Examiner U.s. C 1. X.R.I 

